My understanding of intercessory prayer has more to do with the idea that we need others with a pure conscience to pray for things we're not able to see in ourselves or ask God with confidence, but I think it makes more sense with a living person than with a saint in heaven. Why? Well, if you can believe and ask for prayer for a saint in heaven to intercede then you have enough faith in the supernatural to believe in some spiritual world and ask God, as far as I'm concerned. For someone doubting their faith, but yet inspired by the continuing faith of others and asking them that if there is a God, that they'll pray for them when they are deficient in trust or faith, that makes a difference. We pray for the same things over and over again in our lifetime because we don't generally have enough strength as humans to trust in God all the time, so even if, say, I prayed for a relative's salvation over a decade again, but still pray for it again on and on so long as it hasn't come to pass, it's because I am regenerating or renewing my faith in God's fulfillment of something I am continually longing for.